In IETF quic, start peer migration by switching peer address as soon as detecting a non-probing frame in the incoming packet rather than after finishing parsing the whole packet. Protected by --gfe2_reloadable_flag_quic_start_peer_migration_earlier. Doing so will fix the bug in IETF QUIC that a server may send in-cache response to old address while the end of current packet processing isn't reached PiperOrigin-RevId: 328429809 Change-Id: Ib07274bdef3944240d43defc4183ac03cdd47bba